wiring two pumps/valves on CH/HW system

[chris French]

I've been modifying our CH/HW system. Converting the gravity HW system to pumped. With a new separate pump and 2 port valve for the HW and a zone valve on the CH circuit. (I decided that because of the way the system plumbing is arranged it was easier with a second pump.

Anyway get to wiring it up and realised I'd had a bit of a brainfart when planning it, (that I couldn't of course use the switched output from the 2 port valve to fire the boiler and switch on just the relevant pump, as the other pump would start)

So, I've wired it so that the stats switch on the relevant pump, and the valve signals to the boiler to fire. So the boiler won't fire if the valve doesn't open. All seems to all be working ok.

Now, it does mean that the pump can run with the valve not open. Are the pumps likely to be unhappy with this? Bearing in mind that it's not going to be running indefinitely as we would notice that the heating or HW wasn't working. I could modify things to switch the pumps via relays if necessary.

Any thoughts/experience?

[Davey]

The energy has to go somewhere, and so it builds up as heat, so the pump will get hotter and hotter. What happens then will depend on how long you leave it running like that. In principle, it's not a good idea, but the time taken before damage, if any, occurs is difficult to predict. It might strain the seal, as well.

[ARW]

If the motorized valve is wired in parallel with the pump. then the pump cannot run with the valve closed.

ie call for CH passes through the room stat and then feeds both the CH motorized valve (the brown wire) and the CH pump. The micro switch then supplies the call for heat to the boiler. Repeat for HW.

[John Rumm]

There will be a few secs where the pump is working against a partly closed valve, as it takes a short while to open. Ought not cause the pump any problem, although might cause pumpover depending on the layout of the pipework.

[ARW]

for Chris.

And I suppose a bypass would help.

[Brian Gaff]

Cavitations risk, perhaps?

Brian

[The Natural Philosopher]

use a relay to cerate a wired 'OR' for the system

Its what I had to do with a UFH circulation pump.

UFH valve cuts in circulation pump AND drives a relay whose output is in paralel with all the other motorised valve stuff.

So UFH can call for boiler heat, but other calls for heat don't start the UFH pump.

I built it all in a plastic DIN rail box 'wiring centre' screwed to the wall - a DIN relay was a couple of quid. DIN style connector blocks sort out all the thermostats motorised valves and other wiring..

It was a joy when I added an extra thermostat too.

[chris French]

Ah, thanks I seemed to have missed that. And thanks to other comments as well.

At the moment everything seems to be working ok and I've got a bathroom refurb to crack on with. So I'm going to leave it whilst I decide what to do and get on with that as I'm slipping way behind schedule

[ARW]

Thinking about it then you could use a on delay timer for the pump instead of a bypass if there is a problem

[Bob Minchin]

Also bear in mind that your boiler might need a pump overrun period to cool the heat exchanger. However if the boiler is of the age to have gravity DHW, then this might not be an issue.

[John Rumm]

If that were the case then the pump would usually be powered directly from the boiler and not via the wiring loom.

Its worth adding to old systems though, since it does give a decent improvement in efficiency when the boiler cycles - rather than all that heat in a big block of cast iron and a couple of gallons of water slowly wafting out the flue, it will at least get dumped into the house.

Perhaps we should do some revised wiring diagrams for including overrun timers into them?

[ARW]

[fred]

A pipe stat in place of the timer might be a nice idea, cheaper and less to go wrong.

[chris French]

It's an Ideal Mexico circa 2002, nice big cast iron heat exchanger and a total lack of any fangledness at all :-)

Hmm, might consider that when I've enough roundtuits.

[John Rumm]

ok, let me rephrase that, perhaps we should update the wiki to include the diagrams that we already have in the detailed doc!

[Dave Liquorice]

Timers, what's wrong with pipe stats?

Timer may be set too long and start cooling the HW and/or CH system.

Also need to make sure the excess heat is only dumped into the place that was demanding heat. You probably don't want to dump into the CH in the middle of summer...

[ARW]

But unless you were calling for CH then that will not happen